Superheterodyne receiver



April 26, 1938.

J. K. JOHNSON SUPERHETERODYNE REEIVER Original Filed May 22, 1934 FREQUE/vcyf/L ocra f5) INVENTOR.

J /7//1/ Kauf o/s//VJo/v ATTORNEY.

Patented Apr. 26, 1938 UNITED STATES sUrEaHE'rERoD-YNE RECEIVER.

John Kelly Johnson, Kenilworth, Ill., assigner' to Hazeltine Corporation, Jersey City, N. J., a corporation of DelawareV Original application May 22, 1934, Serial No. 726,931. Divided and this application .uly 12,

1935, Serial No. 30,999

6 claims. (o1. 25d-20) This invention relates to the reception and selection of modulated carrier signals, and has for its principal objects to facilitate the selection of such signals and to improve the delity of reception thereof.

This application is a division of my application, Serial No. 726,931, filed May 22, 1934, for Superheterodyne receiver which issued March 9, 1937 as Patent No. 2,073,344.

A radio broadcast signal is ordinarily transmitted on a carrier wave'having two side bands of modulation frequencies, which are of about 6 kilocycles in width on either side ofthe carrier. Under present broadcasting conditions, the different carrier frequencies are allocated at Various positions throughout the broadcast range, usually 10 kilocycles apart, and in many instances. the side-band frequencies of one signal channel either overlap those of an adjacent signal channel or else closely encroach upon them. In either case, it is difcult, when tuning aradio broadcast receiver to a desired signal in one such channel, to eliminate interference due to signals in the adjacent channels, particularly when such interfering signals are received with a strength comparable to that of the desired signal.

Operation without interference in such cases ordinarily requires that the selecting system shall pass a considerably narrowed band of frequencies so that passage of the interfering signals is substantially prevented. Narrowing the selected band in this manner ordinarily impairs the delity of reception of the signals, since the side-band frequencies corresponding to the higher audio frequencies oi' modulation are suppressed.

It is well known that in the operation of superheterodyne receivers, there is produced an intermediate-carrier frequency which is equal to the difference between the signal-carrier frequency and the local oscillator frequency. This intermediate, or difference, frequency is ordinarily maintained constant over the tuning range of the signals by operating the signal-selecting circuits and the oscillator frequency-determining circuit by a unicontrol device'to maintain the constant difference. The two side bands of modulation which extend on either side of the signalcarrier frequency likewise extend on either side of the intermediate-carrier frequency. It is the usual practice in the case of superheterodyne receivers to locate the principal band-width-deter mining circuits' in the intermediate-frequency portion. Hence, the narrowed band width mentioned above is passed by a highly selective intermediate-frequency selective system.

According to one of the features of the invention described in my above-identified patent, No. 2,073,344, it is proposed to improve the fidelity of reception and yet maintain the narrowed band selective characteristic of the selective circuits. 5 This is .accomplished by utilizing in a superheterodyne type of receiver a local oscillator system, provided with means for shifting the oscillator frequency relative to the signal frequency.

In accordance with this invention, there is proio vided a. selective system which is adjustable to pass any band width between a moderately selective width and a more highly selective width. The adjustable feature is provided by varying the coupling between a pair of coupled tuned l5 circuits, and simultaneously varying the resistance introduced into at least one of these tuned circuits. Since a change of resistance in a tuned circuit also changes the gain, or transmission efficiency, the varying coupling is arranged to automatically maintain the transmission eiliciency substantially constant while the resistance is being changed.

Another feature of the above-identified Patent No. 2,073,344, resides in a cooperation between the two foregoing features, in accordance with which means is provided for simultaneously shifting the local oscillator frequency and varying the selectivity of the tuned selecting system.

Fig. l illustrates a superheterodyne radio receiver embodying all the features of this invention;

Fig. 2 illustrates graphically the expansion of the band selected by a tuned coupling system according to the invention; and

Fig. Bshows the impedance variation of one of the circuits of the tuned coupling system in passing to the expanded condition.

Fig. l illustrates a superheterodyne receiver comprising in tandem in the order named:V an antenna, i0 and ground il; a radio-frequency amplifier I2; an oscillator-modulator I3; an in.- termediate-frequency amplifier i4; and a detector and audio-frequency amplifier, and loudspeaker l5. The devices i2 to l5 are indicated 45 only in generalized block form since they constitute no part of this invention. c n

There is arranged according to this invention an intermediate-frequency coupling system 32, connected between the `output of intermediate- 15.0 frequency amplifier i4 and the detector and audio-frequency amphi-ler, and loudspeaker l5. Means is associated with this coupling system to permit the width of the band transmitted thereby to be varied to change the selectivity and fidelity.

Y' Thislmeans maybe used in conjunction with Vthe oscillator frequency-shifting varrangement of my said copendingapplication. The coupling system Y 32 comprisesc'aVV primary coil 33 electromagneti-Y cally coupled to a secondary coil 34.* 'Irtvhe coil 33 t' is tuned to the.intermediate-carrier frequencyA by a condenser and coil 34 is likewise ituned `to the intermediate-carrier 'frequency by a con;

Y -denser=36. Y f' k f denser 36 an arrangement of a coil 31 and Ya volt- Y ageV divider comprising two Aresistors ,738V and 39. switch is provided Vto connect coil 31 across either .resistor 38 or39-according .toV whether switch'40 makes contact with'switch-point 4| V.or

switch-point 42.. 'I'he Vcoil 31 is electromag-V netically coupled to primary coil 33.

divider' contact arm 43 isconnected to a point 44,.and, when in the neutral position, makes contact .with the neutral point 'between'the two vresistors. In this position, the low potential ends 'of coil 344 and condenserA 36 are directlyconnected.YV

i The switch 40 is correlated to potentiometer arm i but Ywhen arm 43 is4 operatingon resistor 3 9, switchV 43so that when thearm 43 is operating on re-V sistor138, switch '4U makes contact with pointY 4l;

40:makes contact with point 42..Y

1 As arm 43. is moved in either direction from the neutral position; along" resistor 38 or lresistor 39,

. more voltage is introduced intothe secondary circuit'ffromV the coupling between coils A31v and A33,

sincethe voltage acrosscoil 3'! also appearsV across Ythe voltage divider resistance. At the same time,

more vresistance is introduced 'in .the jsecondary circuit from the voltage. divider. The amount of coupling and resistance increase in' theV resonant secondary circuit depends upon howlfar the arm 43.is moved fromneutral. Y 'i Y It is'weufknow that whenythe Vemailing between Ya pair of. coupled `tunedfcircuts is small, the

resonance characteristic of the system is'sharp.

g But when the coupling; is increased above the A* panded by movementl ofthe arm 43. ary circuit would beicharacterized by two Vprosingle'resonantfrequency. Y Y Y t In this manner, then, the selectedband isrex- The second-V nounced V'resonantpeaks; `whenrin the expanded condition, if itwere not for the resistance which Visintroduced TheV effect vof .simultaneouslyV introducing resistance and ,increasing the "coupling,

is-'to expand' the band without producingfobjec" tionable resonancepeaks in theV secondary circuit.

The',double-tunedV "coupling system32 :is Vadvvjustedso'that Ywhenarm 43 is`inV the neutral posi# 'Y t tion (no-resistance being introduced),theselec-VY tivity is quite sharp. typical selectivity curve. VVV'for thisY condition is kcurve ,E ofv Fig.. 2,5Which' is a j graph of attenuation plotted against frequency. Y YIn the example illustrated,gthe intermediate-carrierfreduency is F-kilocycles. YThe selectivity Y"represented bycurve E issuch as to pass a band V E'fprovides high selectivity, theiidelity is poor.

ifi..

i 1.7761about,4:ki1ooyciesinwidth; thats, each sido j c Vrierfrequency of l'kilocy'cles. f

band passedlisi2j kilo'cyclesin4 width, for a car- Y While the characteristic represented by curve, To

improve theiidelity. then,thevoltage divider arm Therefis arranged infv series between'the lowerY Y endsV (the low potentialfends) of coil 34 andcon- The voltage 434is moved from theY neutral position to increase Y the coupling and-to introduce resistance into the 1 tuned circuit, as explained above.` When the f arm- 43 is rotated the V.maximum distance inyone A f direction Vor the otherg'the transmissionicharacteristic takes the form of curveF in Fig.V 2.

According tocurve F, coupling 'system' 32 'selects' a band about 8 kilocyclesin'width, from 171 to 179 kilocycles. f' f If it were attempted to expand theband -by the addition of resistance alone, the characteristic would take a `form somewhat like curve G of VI'ig'. 2.

Curve G shows: greater attenuation than curve E within the band, thisbeing due to the reduction of Y transmission'efficiency'because of the added re.

sistance. Curve F, however, shows less attenuation than curveG, and shows about the same attenuation .as curve E. The difference inY attenuation'lbetween curves G and Fis made up by Thusthefv gain due to coupling coil 31Ycompensates thelosrs,

the, additional couplingdue to coilv 3l.

due tothe resistance.

It should be understood thattho Varrangoinorlt y For example, if it be Vfound that the primary cirof the inventionY is readily susceptible -to l Y Yvariations from the specic embodiment described.

cuit 33, 35can stand being loaded by both resistors 38 and 39 in parallel, the switch 40 may fbe dise pensed with and a permanent*co-nnectionmay be Vmade between points 4| Vand 42 and the point to which the switch is' shown connected.'Y Y

'Ifnere is associated with the receiver aV system of automatic volume control. Thissystemc'o-m'- prises a path 46 .between a-point 41 of coil 33 and' Vapoint 481which is connected to theV signalgridr circuitsV ofY tubes of the':intermediate-frequency Y amplifier I3.

There `is included 'in `the path'46 i automatic Volume controlapparatu's designatedA4 i ,generally by the block 56. This automatic volume control apparatus maybe of any conventional form whichordinarily includes a rectifier for Vde-fveloping a unidirectional Voltage which biases theY Y grids of tubes of the amplifier I3 inverselywith f any change'of the intermediate-.frequency voltage at coil33 and suitable filters.

This arrangement, in addition to automatically i Y vcontrolling the volumeoutput, producesV therdesirable effect of `'maintaining a substantially` uniform output withexpansion. This eiect is due-g to the manner'in' which the impedance of the'.`

primarycircuit 33, 35 changes in shifting from:v

Athecontracted tctheexpanded condition'of coupling system 32. v Y o Y Impedancerv characteristics ofthe primary cir,-Y

ycuit 33, 35 are'iliustratedV in Fig. Y3 in which `the impedance is. plotted against thev intermediate VVfrequency. Curve' K illustrates the relatively Y Vsharp impedance curve'obtained when Vthe band passed bythe coupling system isV contracted'by placing the arm 43 inthe neutralposition. Whenthe band passed bythe coupling Vsystem is' exV YVpanded, as byr1mov'ing the arm'43 inV either Vdirection',V and' the coupling betweenthe primary fand secondary circuits ,ofdthe'coupling system thereby increased,V the impedancev atl Athe primary circuit becomes characterized by the doubleres; Y onance eiect due to over-optimum coupling.Y

expandedcondition is shown by curve L, from pronounced peaks at the Yprimary circuitwhile Vsuch peaks are absent at the secondary circuits The impedance of the primary circuit under Ythe l' whichit is 'observed that Vthere are tWo` proLy nounced resonant pea-ks anda dip'betweenV themy at thefcarrier frequency. Y The'reason for the;

lies in the fact that no additional resistance is introduced into the primary circuit when the coupling is increased.

Since the voltage applied to the automatic volume control apparatus is taken from the primary circuit, the automatic volume control effect is caused to relax when the band width selected by coupling system 32 is expanded. The reason for this is that the intermediate-carrier frequency lies within the dip in curve L. In consequence the volume output rises somewhat when the band width is expanded; or, in other words, the sensitivity would be somewhat less in any except the expanded condition.

This counteracts the tendency for the volume to drop due to the moving of the carrier to one side of the sloping resonance characteristic and thus decreasing in amplitude as the band is expanded. This also compensates for the net drop in output which results from the cutting olf of a large portion of one of the side bands.

1f the receiver is tuned in the expanded condition, this primary impedance characteristic produces a noise suppression and sharp tuning effect. On either side of the exact resonance frequency the voltage actuating the automatic volume control is abnormally high, thus decreasing the receiver gain, while at exact resonance the automatic volume controlV voltage is abnormally low. This condition produces a rapid and easily distinguished output volume rise when tuning through resonance, which indicates the exact point at which the receiver is correctly tuned.

What is claimed is:

l. A carrier-frequency coupling system adapted to select a band of frequencies including a carrier frequency and associated sidebands, comprising a pair of coupled circuits, each of which is tuned to said carrier frequency, and means for increasing the width of said band which com- Y prises auxiliary means coupling said circuits, an

adjustable resistance element connected in circuit with said auxiliary means and having at least a portion thereof effectively included in one of said circuits, and means for adjusting said element to increase the resistance of said one of said circuits and simultaneously to increase the coupling between said circuits by said auxiliary means.

2. A carrier-frequency coupling system adapted to select a band of frequencies including a carrier frequency and associated sidebands, comprising a pair of coupled circuits each of which is tuned to said carrier frequency, means for expandlng said band which comprises an auxiliary element included in a first of said circuits and coupled to the second of said circuits, and a resistance voltage divider including an adjustable contact arm, connected in circuit with said auxiliary element and having at least a portion thereof effectively included in said rst circuit, adjustment of said arm being effective to increase the amount of resistance included in said first circuit and simultaneously to increase the coupling between said rst and second circuits due to said auxiliary element.

3. A carrier-frequency coupling system adapted to select a band of frequencies including a carrier frequency and associated side bands, comprising a transformer having a primary and a secondary coil each of which is tuned to said carrier frequency by capacity, and an auxiliary coil, shunted by a voltage divider, in series with one of said transformer coils and its associated capacity, said auxiliary coil being coupled to the other of said coils, whereby adjustment of said Voltage divider changes the resistance in the circuit of said one of said coils and also changes in the same direction the coupling between the circuits of said primary and secondary coils.

4. A carrier-frequency band-selecting system comprising primary and secondary circuits electromagnetically coupled, each of said circuits being tuned to said carrier frequency, an auxiliary coil included in said secondary circuit and coupled to said primary circuit, and a Voltage di-d vider connected across said auxiliary coil and including a contact arm adjustable to vary the coupling due to said auxiliary coil and simultaneously to vary the resistance introduced into said secondary circuit in the same sense, whereby the selected band may be expanded by increasing the coupling and the resistance in said secondary circuit.

5. A carrier-frequency band-selecting system comprising primary and secondary circuits electromagnetically coupled, each of said circuits being tuned to said carrier frequency, an auxiliary coil included in said secondary circuit and coupled to said primary circuit, a voltage divider having two connected resistors and a movable contact arm which when in the neutral position contacts with a point between said resistors, switching means for connecting either one or the other of said resistors across said auxiliary coil, and means for automatically operating said switching means to connect across said coil whichever resistor said contact arm makes contact.

6. In a radio receiver, a carrier-frequency band-selecting system comprising a primary and a secondary circuit coupled together, each of said circuits being tunedV to the carrier frequency, means for increasing the coupling between said circuits and for simultaneously increasing the resistance introduced into said secondary circuit, whereby when said coupling is increased the Vselected band width is expanded and Vsaid. primary circuit becomes characterized by pronounced resonance peaks near each side of said selected band but said secondary circuit remains substantially free from resonance peaks, and an automatic volume control arrangement for controlling the output of said receiver, the input of said varrangement being connected to said primary circuit, whereby the automatic volume control effect relaxes when the selected band width is expanded.

JOHN KELLY JOHNSON. 

